When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Various embodiments of the present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as “spin-warp” and results in Cartesian k-space sampling. The spin-warp technique is well-known and employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then an NMR signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
In an NMR imaging sequence, a uniform magnetic field B0 is applied to an imaged object along the z axis of a Cartesian coordinate system. The effect of the magnetic field B0 is to align the object's nuclear spins along the z axis. In this field, the nuclei resonate at their Larmor frequencies according to ω=γB0, where ω is the Larmor frequency, and γ is the gyromagnetic ratio which is a property of the particular nucleus. The nuclei respond to RF pulses at this frequency by tipping their longitudinal magnetization into the transverse, x-y plane. Water, because of its relative abundance in biological tissue and the properties of its proton nuclei, is of principle concern in such imaging. The value of the gyromagnetic ratio γ for protons in water is 4.26 kHz/Gauss and therefore in a 1.5 Tesla polarizing magnetic field B0, the resonant or Larmor frequency of water protons is approximately 63.9 MHz.
Materials other than water, principally fat, are also to be found in biological tissue and have different gyromagnetic ratios. The Larmor frequency of protons in fat is approximately 210 Hz lower than that of protons in water in a 1.5 Tesla polarizing magnetic field B0. The difference between the Larmor frequencies of such different isotopes or species of the same nucleus, viz., protons, is termed chemical shift, reflecting the differing chemical environments of the two species.
In the well known slice selective RF pulse sequence, a slice select magnetic field gradient Gz is applied at the time of the RF pulse so that only the nuclei in a slice through the object in an x-y plane are excited. After the excitation of the nuclei, magnetic field gradients are applied along the x and y axes and an NMR signal is acquired. The readout gradient Gx along the x axis causes the nuclei to precess at different resonant frequencies depending on their position along the x axis; that is, Gx spatially encodes the precessing nuclei by frequency. But because water and fat spins resonate at different frequencies, even when they are in the same location, their locations in the reconstructed image will be shifted with respect to each other. This is particularly problematic on the boundaries of tissues or organs where this chemical shift can cause blurring or multiple edges.
There is a large body of art that has been developed to suppress the signals from either water or fat. Reliable and uniform fat suppression is essential for accurate diagnoses in many areas of MRI. This is particularly true for sequences such as fast spin-echo (FSE), steady-state free precession (SSFP) and gradient echo (GRE) imaging, in which fat is bright and may obscure underling pathology. Although conventional fat saturation may be adequate for areas of the body with a relative homogeneous B0 field, there are applications in which fat saturation routinely fails. This is particularly true for extremity imaging, off-isocenter imaging, large field of view (FOV) imaging, and challenging areas such as the brachial plexus and skull based, as well as many others. Short-TI inversion recovery (STIR) imaging provides uniform fat suppression, but at a cost of reduced signal-to-noise ratio (SNR) for the water image and mixed contrast that is dependent on T1, (Bydder G M, Pennock J M, Steiner R E, Khenia S, Payne J A, Young I R, The Short T1 Inversion Recovery Sequence—An Approach To MR Imaging Of The Abdomen, Magn. Reson. Imaging 1985; 3(3):251-254). This latter disadvantage limits STIR imaging to T2 weighted (T2W) applications, such that current T1 weighted (T1W) applications rely solely on conventional fat-saturation methods. Another fat suppression technique is the use of spectral-spatial or water selective pulses; however, this method is also sensitive to field inhomogeneities, (Meyer C H, Pauly J M, Macovski A, Nishimura D G, Simultaneous Spatial And Spectral Selective Excitation, Magn. Reson. Med. 1990; 15(2):287-304).
A water and fat separation technique, “In and Out of Phase” Imaging was first described by Dixon in 1984, and is used to exploit the difference in chemical shifts between water and fat in order to separate water and fat into separate images, Dixon W. Simple Proton Spectroscopic Imaging, Radiology 1984; 153:189-194. Glover et al further refined this approach in 1991 with a 3 point method that accounts for magnetic field inhomogeneities created by susceptibility differences, Glover G H, Schneider E, Three-Point Dixon Technique For True Water/Fat Decomposition With B0 Inhomogeneity Correction, Magn. Reson. Med. 1991; 18(2):371-383; Glover G, Multipoint Dixon Technique For Water and Fat Proton and Susceptibility Imaging, Journal of Magnetic Resonance Imaging 1991; 1:521-530. Hardy et al first applied this method with FSE imaging by acquiring three images with the readout centered at the spin-echo for one image and symmetrically before and after the spin-echo in the subsequent two images, Hardy P A, Hinks R S, Tkach J A, Separation Of Fat And Water In Fast Spin-Echo MR Imaging With The Three-Point Dixon Technique, J. Magn. Reson. Imaging 1995; 5(2):181-195.
Recently, Jingfei Ma described an improvement on the original two point technique described by Dixon, Ma J. Breath-Hold Water And Fat Imaging Using A Dual-Echo Two-Point Dixon Technique With an Efficient And Robust Phase-Correction Algorithm, Magn. Reson, Med. 2004; 52(2):415-419. In this method, two echoes that are in-phase and out-of-phase are acquired, just as in the original description by Dixon, but an upwrapping algorithm is used to unwrap ambiguities between water and fat, to remove water-fat “swapping” that can occur in the presence of field inhomogeneities. This method has also been extended to a 3D-SPGR acquisition where the two readouts are acquired in the same pulse sequence, or TR, with the readout gradients having opposite polarity, Ma J, Vu A, Son J, Choi H, Hazle J, Fat-Suppressed Three-Dimensional Dual Echo Dixon Technique For contrast Agent Enhanced MRI, J. Magn. Reson. Imag. 2006; 23:36-41. This water-fat separation appears to work well, but it is important to note that high bandwidths are used such that chemical shift artifacts may be problematic for this approach at lower bandwidths. It is very advantageous to image with lower bandwidths; to acquire images with higher signal to noise ratio (SNR), improving the quality of the images. Limited SNR also results in limited resolution. Further work by Ma, such as that presented at the 2006 ISMRM describes a three-echo method that uses alternating polarity gradients. However, the images presented in this work show clear evidence of the effects of chemical shift artifact related to the alternating readout gradient polarity.
It has been observed that when multiple images are acquired during a single pulse sequence, or TR, using a readout gradient of alternating polarity, that the images are not spatially aligned with each other as a result of chemical shift artifact. As a result, when the images are combined using one of the above described methods, artifacts such as blurring or double edges occur at the boundaries of tissues and organs even in images that depict only water or fat spin density. This artifact is particularly troublesome at higher B0 field strengths where chemical shift is larger or when the receiver bandwidth is reduced in order to improve the signal to noise ratio (SNR).
Also recently, a water-fat separation method known as IDEAL (Iterative Decomposition of water and fat with Echo Asymmetry and Least squares estimation) has been described by Reeder S, Pineda A, Wen Z, et al., in Iterative Decomposition of Water and Fat With Echo Asymmetry and Least-Squares Estimation (IDEAL): Application with Fast Spin-echo Imaging, Magn Reson Med 2005; 54(3);636-644. IDEAL is an SNR-efficient method that uses flexible echo spacings and when combined with optimized echo spacings can provide the best possible SNR performance. In addition, IDEAL can easily be extended to systems with multiple species such as C-13 labeled pyruvate. Reeder S, Bittain J, Grist T, Yen Y, “Separation of C-13 Metabolites with Chemical Shift Imaging”, 2006; The International Society of Magnetic Resonance 14th Meeting.
Others have combined water-fat separation methods with non-spin-warp imaging, including spiral imaging and 3D-projection reconstruction (VIPR). In spiral imaging, chemical shift results in blurring of the image. Simply separating the water and fat signals will not remove this blur. Correction for blurring from off-resonance fat shifts in spiral imaging have been reported, but are image-based and computationally intensive. A simpler method is needed to remove the effects of the fat phase shifts that lead to blur.